The disclosed embodiments of the present invention relate to a power detector and associated method for detecting the output power of a power amplifier.
A power detector is utilized for performing a transmission signal strength indication (TSSI) operation in order to perform transmission power compensation. The power detector is required to be less sensitive to transmitter output impedance variation. Please refer to FIG. 1, which is a diagram illustrating a conventional power detector 100 arranged for detecting the output power of the power amplifier 30. The power detector 100 includes a power amplifier replica 40, a multiplier 50 and a detection module 60. The power amplifier 30 amplify input voltage Vi and output amplified current I1, and the power amplifier replica 40 receive the same input voltage Vi, and output replica current I2 for detection, respectively, wherein I1=Gm*Vi, I2=a*Gm*Vi, and “a” is replica ratio (a constant parameter) of the power amplifier replica 40. The output voltage V1 equals to Z*I1, wherein Z is the impedance value of the antenna 72. Since the current I2 is the detected replica current, the detected power Pdet may be obtained by multiplying the detected voltage V1 and the detected current I2.
There exist phase delay between transmission and detection in both voltage and current paths. We can simplify it the equivalent phase delay as “φ” exist in voltage detection path only as shown in FIG. 1, and the I-V phase difference caused by non-ideal matching of the antenna 72 is denoted as “θ”, wherein if the antenna 72 is an ideal antenna having the nominal 50Ω impedance, the phase difference θ introduced by the antenna 72 is zero.
The accuracy of detecting the power of the power amplifier 30 is affected and limited by both the phase delay φ and the phase difference θ. The output power Po of the power amplifier 30 is represented as Po=(Gm2Vi2|Z|cos θ)/2, and the detected power Pdet is represented as Pdet=aGm2Vi2|Z|cos (θ+φ). The ratio of the detected power Pdet and the output power Po is represented as Pdet/Po=2 a cos (θ+φ)/cos θ. Since “a” is a constant, the value of the ratio “cos (θ+φ)/cos θ” may be used to indicate the difference between the output power Po outputted by the power amplifier 30 and the detected power Pdet. If the phase delay φ is too large, the output power Po will not be followed by the detected power Pdet, resulting in the phase difference θ unable to be tracked. Consequently, the detected power Pdet would be unable to track the variation of the output power Po which is caused by non-ideal antenna.
Therefore, there is a need to provide a novel power detector capable of compensating the phase difference between the transmission path and the detection path of the power amplifier.